The present invention is related generally to radio communication devices and, more particularly, to the improvement of distortion characteristics of amplifiers within the transceivers of radio communication devices.
Selectivity filters have conventionally been used in radio communication device transceivers to enable the transceivers to xe2x80x9ctunexe2x80x9d to a desired channel while rejecting other adjacent channels. Historically, selectivity filters consisted of discrete RLC components or Surface-Acoustic-Wave (SAW) devices. These types of selectivity filters were generally linear and thus did not significantly contribute to the inter-modulation distortion or 1 dB compression point (P1dB) of the overall transceiver. Inter-modulation distortion occurs when two interferers at two adjacent channels generate an inter-modulation component which falls into the selectivity filter band. As is known in the art, the third order intercept (IP3) of the filter provides a standard measure of the inter-modulation performance of the circuit. The 1 dB compression point (P1dB) represents a measure of the linearity of the filter and is the point at which the difference between measured output power from the filter is 1 dB below what would be expected due to a corresponding increase in input power to the filter.
Recently, selectivity filters such as Intermediate Frequency (IF) filters, along with associated driving amplifiers, have been relocated onto the same Application Specific Integrated Circuit (ASIC) as other transceiver circuitry. Integrated IF amplifier and filters, however, generally have degraded distortion performance. An IF amplifier 105 driving an integrated IF filter 110 in a conventional integrated transceiver circuit 100, such as that shown in FIG. 1, typically has an IP3 10-14 dB higher then the. P1dB. In the integrated IF filter 110, however, the difference between IP3 and P1dB can be as high as 20 dB. Therefore, the IF amplifier 105 limits the distortion characteristic of the combination of the IF amplifier 105 and the IF filter 110. The IF amplifier 105 thus significantly contributes to the overall transceiver distortion. In order to optimize the overall performance of an integrated transceiver, it would therefore be advantageous to reduce the distortion contributed by the IF amplifier which drives the IF filter.
The above described desirable characteristics and others are provided by the following exemplary embodiments of the invention.
According to one exemplary embodiment of the invention, an amplifier is provided. The amplifier of this exemplary embodiment comprises: a first differential stage that compares a first voltage with a second voltage and generates a first output current; a first current mirror having an input coupled to receive the first output current and an output that supplies a second output current; and a first output transistor having an input coupled to receive the second output current, and an output for supplying a third output current for driving a load.